This invention relates to distance measuring equipment (DME) for assisting in the navigation of an aircraft along a desired path, especially DME employing interrogators and transponders and, more particularly, to DME wherein there is required a high degree of accuracy as compared with conventional systems.
Distance measuring equipment (DME) and distance measuring techniques are widely accepted as a navigation and landing aid. The distance of an aircraft from a selected ground beacon is determined by measuring the round trip time of travel of radio pulse signals between the aircraft and the ground beacon. Easy-to-read meters are used on the aircraft to display the distance readings. A ground based radio transponder or beacon produces artificial echoes within a frequency channel which positively identifies the source of the "echo" and hence the geographic location of the transponder. An airborne interrogator containing a transmitter repeatedly initiates the distance measuring process by sending out widely spaced interrogation pulse pairs. The interrogation pulses are then received by the ground beacon, which beacon contains an associated transmitter for sending out reply pulses. The replied pulses are finally picked up by the airborne receiver, and timing circuits automatically measure the round trip travel time or interval between interrogation pulse transmission and receipt of reply pulses. This time interval is then converted into electrical signals which reflect distance from aircraft to beacon on a distance meter.
Since clear frequency channels are used, a ground beacon is not blocked or overloaded by interrogation pulses which are intended for a different beacon, i.e., a beacon receiver will respond only to interrogation pulses that are intended for it, since all other DME interrogations within a given geographical area occur on a different radio frequency.
Also, in the aircraft navigation field there are already known instrument landing systems (ILS) and techniques for use in guiding an aircraft onto a particular airport runway. The approach and landing zone encompasses an area having a radius of approximately 25 to 30 miles. In this zone, the overriding requirement is an extreme degree of precision navigational guidance in order to bring the aircraft safely down on a lateral course aligned exactly with the runway and a path of descent of suitable angle leading to the touchdown point. One known approach employs two approach beacons, one of which defines a guide course in the azimuth plane to direct the aircraft along the runway (localizer beacon) while another defines a glide path course which will guide the aircraft down to a landing point on the runway (glide path beacon).
In the conventional L Band DME systems, standardized by the International Civil Aviation Organization (ICAO), the interrogations and replies comprise pairs of Gaussian pulses, each of 3.5 microseconds duration, with rise times of the order of 2.5 microseconds. The resulting accuracy, limited by this rise-time, is less than what may be desirable for the most stringent applications, such as those associated with the Microwave Landing Systems (MLS). The proposed microwave landing system (MLS) suggested by a special committee of the Radio Technology Commission for Aeronautics requires associated distance measuring equipment which operates on channels adjacent to the angle measuring channels in the 5.0 to 5.25 GHz band (C-Band). 200 channels for each service are required, i.e., 200 DME channels and 200 very high frequency omni range (VOR) bearing channels with 40 VOR channels reserved for ILS. However, many potential users have expressed objections to carrying an extra DME interrogator which would be used only during the last few minutes of flight (during landing) if there is a possibility of using the existing L-Band DME interrogator presently paired with VOR and ILS.
The heaviest use of DME is in the United States where there are approximately 700 DME transponders and approximately 70,000 L-Band DME equipped aircraft. Since each transponder is designed to handle 100 aircraft, the system would appear to be saturated. However, this would only be true if all aircraft were in the air at the same time and all transponders were within the same line-of-sight area. This is not the case. On the average, there are approximately 3200 aircraft in flight in each of ten line-of-sight areas of approximately 600 miles in diameter. Therefore, the average number of aircraft per area is 320. Within each such area, there are on the average 70 transponders. However, there is a potential for 200 transponders since there are 200 channels. The present average loading is therefore 320/70 = 4.5% of capacity, and the potential average loading for 200 transponders is 320/200 = 1.6% of capacity. It is conceded that peak loading may be considerably greater. Current estimates are that 16,000 aircraft may be simultaneously in flight in the United States and that some transponders may reach 60% of capacity. The fact remains, however, that the average loading is light and that this fact can be exploited since MLS is used only during the landing of aircraft, and an aircraft using an L-Band DME is not simultaneously using an enroute or ILS DME.
The International Civial Aviation Organization (ICAO) has required an accuracy of 0.5 mile or 3% of distance, whichever is greater. However, this requirement reflects hardware of 25 years ago, and there hardly exists today any combination of interrogators and transponders which do not exhibit also an order of magnitude improvement in accuracy.
Using the standard 3.50 microsecond Gaussian pulses, it has been demonstrated that accuracies of .+-. 80 ft. over a signal level change of 60DB are achievable. Further, Tacan sets have been delivered which exhibit a .+-. 40 meter accuracy. These increased accuracies are attributable to (a) digital ranging circuits with accurate clocks; (b) "pilot pulses" in both interrogators and transponders which measure and calibrate out internal interrogator or transponder delay; (c) Instantaneous Automatic Gain Control (IAGC) which employs one-half amplitude finders to reduce delay variations caused by pulse amplitude changes; and (d) first pulse timing to reduce multi-path effects. These techniques are not new but are offered to illustrate means by which C-Band DME systems have achieved a high degree of accuracy. Further, these techniques are well known in the art and a further discussion of them is not deemed necessary.
Recent tests of a C-Band interrogator and transponder calibrated for zero error at touchdown showed a minus 10 ft. error at the far end of the runway and +20 ft. error seven miles from touchdown. This was well within specified limits of .+-. 20 ft. prescribed by the above mentioned special committee. It must be borne in mind, however, that this accuracy was achieved using pulses having 0.1 microsecond rise time. Further, it should be noted at this time that currently used L-Band interrogators employ pulses having a 2.5 microsecond rise time.